As $35 Billion Tie-Up Faces Questions, CEO Says, 'There Is No Plan B'

Omnicom Group Inc. Chief Executive John Wren said it is uncertain when the global advertising giant's proposed $35 billion merger with Publicis Groupe S.A. will be completed, citing unexpected tax issues that could potentially derail the deal.

In the starkest warning so far that serious questions face the giant deal, designed to create the world's largest advertising company by revenue, Mr. Wren told analysts "there is no Plan...